Guidance: What is a hypoxic blackout?
Dec 07, 2021GUIDANCE
Extended, underwater breath-holding by human beings for food gathering is a practice that can be dated back over 2,000 years via sources in Japanese art and literature (Nukada, 1965). Around the world, extended breath-holding still features as part of working and daily life (Thompson, 2016).
Although never contested as a separate sport, underwater swimming was featured as a contested event in the 1900 Olympic Games in Paris (Mallon et al., 2011). The Frenchman Charles Devendeville completed a 60-meter straight-line underwater swim in 68.4 seconds on 12th August 1900. In the 1904 Olympic Games, a plunge distance event involved diving into the water to see who could reach the longest distance (without propulsion) in 60 seconds took place (Wallechinsky, 2012). It was won by William Dickey, who managed 19.05 meters. Both events were never repeated.
Definitions
Various related terms have historically been used to describe hypoxic blackout, including underwater blackout, shallow water blackout, hyperventilation, excessive breath-holding, underwater blackout syndrome, sudden underwater blackout syndrome (SUBS), breath-holding blackout, and free-diver blackout.
At the World Conference on Drowning Prevention in 2011, Dickinson defined shallow water blackout as the "loss of consciousness during an apnoea dive to a depth of fewer than 5 meters preceded by hyperventilation, where alternate causes of blackout have been excluded (Dickson, 2013). Following the conference in May 2011, The International Life Saving Federation published a medical position statement (MPS-16) on shallow water blackouts. It avoided an express definition of the term owing to the "various and numerous" terms in circulation but went on to rely heavily on Dickson's definition:
Swimming underwater whilst breath-holding is safer if effective physical activity continues for the duration the swimmer is underwater, hyperventilation has not been performed prior to entry, no attempt to breathe against a closed glottis occurs and the water is less than 5m deep.
Pearn et al (2015) proposed a revised definition:
We use the terms underwater blackout syndrome or hypoxic blackout. We define the condition as the loss of consciousness in the underwater swimmer or diver, during an apnoea submersion preceded by hyperventilation, where alternative causes of unconsciousness have been excluded.
This definition has attracted significant support, which has persisted to the present day.
Since 2015, recognition of the term "hypoxic blackout" has enabled greater disassociation with its shallow water diving roots. Hypoxic blackout does not correlate with water depth, and it can occur in any depth of water (Boyd, 2015, 518). The term shallow water blackout originated from freediving, where it was used to describe unconsciousness in water typically less than 5 meters either due to equipment failure or prolonged breath-holding. This formed the basis of Dickson's definition in 2011. The American Red Cross, YMCA, and USA Swimming have since redefined their safety guidance to utilize the term "hypoxic blackout" to reinforce the irrelevance of water depth to the event described.
Shallow Water Blackout Prevention (SWBP), a non-profit set up by Rhonda Dawes Milner in the wake of her son's death Whitner Milner to hypoxic blackout in 2011, continues to reinforce the point that extended periods underwater, repeated, with short or little rest is also known to result in a hypoxic blackout, without being preseeded by hyperventilation.
This is an important addition to the 2015 definition and raises two other clarifications:
Whilst underwater swimming is the most common activity preceding hypoxic blackout under the surface of the water, the event can still occur in non-swimmers or those adopting a static position for extended periods before a hypoxic blackout occurs
Repeated submersion during play with small children or synchronized diving (which is distinct from "dunking" during horseplay or "restraint" during competitive sport such as water polo where submersion is imposed against an individual's intent) has the same pathology as hypoxic blackout. Some cases of repeated submersion of children may involve positive intent, immaturity may also contribute to the late identification and ineffective communication that the threshold from playful submersion and resistance to submersion has passed if any warning at all is perceptible by the child.
These three pathologies should not be excluded from the definition of hypoxic blackout. Lifeguard training programs should not teach lifeguards to take reassurance by the material absence of hyperventilation before extended periods underwater. Doing so may delay rescue and increase the risk of harm to the pool user.
These clarifications have largely been adopted in softening the role of hyperventilation in hypoxic blackout cases. The Royal Life Saving Society Australia now refers to hypoxic blackout as "often due to breath-holding or purposeful hyperventilation" before diving (RLSSA, 2021). Although Ryan (2020) continued to adopt the 2015 hypoxic blackout definition published by Pearn et al., he acknowledges that the activity "becomes particularly dangerous when there is little rest between prolonged breath-holding attempts."
The 2015 definition offered by Pearn et al. provides the essential pieces for a modified framework to address these additional risk factors:
Hypoxic blackout describes a loss of consciousness during an apnoea submersion attributable to hyperventilation, extended submersion, or repeated submersions over a short duration in circumstances where alternative causes of unconsciousness have been excluded.
A common problem in explaining fatal drowning accidents during a hypoxic blackout is that the official cause of death is listed as “drowning” (Pollock, 2006). Alternative causes of unconsciousness should include consideration of known causes, including:
- Cardiac arrest.
- Sudden adult death syndrome (SADS).
- Spinal or head injury.
- Seizure.
Causes
It may occur because of:
- Swimming underwater for prolonged periods, which may be proceeded by hyperventilation
- Remaining submerged and largely stationary for long periods
- Repeated and rapid submersion and re-surfacing
Identification
Lifeguards can identify that a casualty is in danger by looking out for signs, including:
- Prolonged periods of submersion
- A lack of purposeful movement over an extended period
- A lack of responsiveness to external events
- Spasms in the arms, legs, or jaw
Hypoxic training describes prolonged periods of submersion as part of a fitness routine. We recommend that hypoxic training only take place in programmed sessions under the guidance of a competent instructor.
Myths
1. Hypoxic blackouts can occur in any depth of water.
2. Hypoxic blackout can occur when stationary and submerged for long periods as well as whilst swimming
3. Hypoxic blackout can occur in swimmers and non-swimmers of any level and ability
Cases involving hypoxic blackout
The case of Madhav Cherukuri is one example of a hypoxic blackout resulting from breath-holding exercises and competitive training.
Madhav Cherukuri was a 25-year-old Staffordshire University international student from India...On Wednesday 30th August 2006...Mr Cherukuri was seen by other swimmers engaging in breath-holding exercises and extended swimming underwater. A fellow swimmer, Ms Elizabeth Hutton, who did not know Mr Cherukuri became so concerned by his breath-holding exercises that she exited the poolside to seek assistance from hotel staff. She exited through the changing rooms and into reception to alert them to her concerns about Mr Cherukuri’s behaviour (Liverpool Echo, 2009).
...When Ms Hutton returned with member of staff, Mr Michael Johnson who was a trained lifeguard, Mr Cherukuri was lying motionless on the bottom of the pool. Mr Cherukuri was later pronounced dead upon arrival at the hospital as a result of his injuries.
Another case example involved a 26-year-old man named Olly Kimber at the Woodlands Caravan Park in Trimmingham:
At around 14:00, he swam 30 lengths in quick succession before climbing out of the pool for a short rest. Mr Kimber then re-entered the pool and swam a further length, when he lost consciousness (Downes, 2020).
Mr Kimber was underwater for around five minutes before he was identified and pulled from the water by four guests at the holiday park who then summoned help from park staff (Downes, 2020; Pringle, 2020). Park staff then provided resuscitation and summoned the emergency services.
References
Boyd, C., Levy, A, McProud T, Huang L, Raneses E, Olson C. (2015). Fatal and nonfatal drowning outcomes related to dangerous underwater breath-holding behaviors, New York State, CDC, 1988-2011. Morbidity and Mortality Weekly Report, 22:64(19), 518-521. Available at: https://pubmed.ncbi.nlm.nih.gov/25996093/ accessed 16th May 2021.
Dickinson, P. (2013). Shallow Water Blackout. In: Bierens, J. (eds) Drowning (Springer, Berlin, Heidelberg). Available at: https://doi.org/10.1007/978-3-642-04253-9_88 accessed 16th May 2021.
Mallon, B., Buchanan, I., and Heijmans, J. (2011). Historical Dictionary of the Olympic Movement. (Scarecrow Press, London), 362. ISBN 9780810875227.
Pearn, J., Franklin, R., and Pedan, A. (2015). Hypoxic Blackout: Diagnosis, Risks, and Prevention. International Journal of Aquatic Research and Education, 9, 342-347. Available at: http://dx.doi.org/10.1123/ijare.2015-0036 accessed 16th May 2021.
RLSSA (2021). Hypoxic blackout. Available at: https://www.royallifesaving.com.au/stay-safe-active/risk-factors/hypoxic-blackout accessed 16th May 2021.
Ryan, M. and Lau, H. (2021). Shallow water blackout. (StatPearls Publishing, Online). Available at: https://www.ncbi.nlm.nih.gov/books/NBK554620/ accessed 16th May 2021.
SWBP. (2020). Shallow water blackout prevention. Available at: http://www.shallowwaterblackoutprevention.org/ accessed 16th May 2021.
Thompson, H. (2016). The ‘sea-nomad’ children who see like dolphins. (BBC) Available at: https://www.bbc.com/future/article/20160229-the-sea-nomad-children-who-see-like-dolphins accessed 16th May 2021.
Wallechinsky, D. (2012). The Book of Olympic Lists. (Aurum Press, London), 300. ISBN 9781845137731.
Citation: Jacklin, D. 2021. What is a hypoxic blackout? Water Incident Research Hub, 7 December; updated 16 June 2022.
